The present invention relates generally to paperboard carton handling and, more particularly, to an apparatus/method for feeding and heat sealing cartons by activation of a coating on the carton to form an adhesive.
It has been known for many years to form paperboard cartons or boxes on a forming machine including a plunger and die arrangement at a first station, fill the carton at a second station through its open top and then close and seal the carton as the cartons are being transported along a series of conveyors. For many years, the most successful approach to sealing of the cartons for closing required a four step process of preheating of the carton and the closing flap by a radiant heater, application of a liquid adhesive to the carton/flap, conversion of the adhesive to establish a high tackiness and then applying pressure between the flap and the carton for the final sealing/closing. It will be apparent that having to carry out three separate and extended operations including heating by radiation prior to the final sealing/closing step, greatly increases the length and cost of the cartoning line, as well as the operating cost including a high electrical power requirement. In addition, this prior technology severely limits the speed by which the cartons could be processed on the cartoning line.
While this prior art process worked reasonably well, it was also limited in its use with prior art conveyors having lugs or flights that are mounted on drive chains engaging the cartons from below and pushing along the rear edge. A good example of one of the most successful cartoning machines and methods utilizing this approach is described and claimed in the U.S. Patent to Gobalet U.S. Pat. No. 2,984,598, owned by the assignee of the present invention. This particular prior art patent illustrates the carton machine/method as it applies to a Charlotte type carton and describes a typical sequence of sealing and closing wherein the front flap is first sealed, and then the two side flaps are sealed in the same manner at a downstream position. Between the two sealing positions, the carton is rotated through 90.degree., as is known in the art.
Over the years, improvements have been made to improve on the Gobalet invention, and in particular to reduce the number of steps required and the length of the machine prior to pressing the flap(s) against the carton. As modern thermoplastic coated paperboard cartons became available, success in sealing and/or carton closing could be accomplished by combining air with the heat source for introduction within the inverted V formed by the flap and the carton during continuous movement along the line. It was found that success could be gained by blowing opposed, high velocity streams of heated air to convert the thermoplastic film to a tacky state to form the adhesive, rather than relying on the relatively slow radiation heat approach. It was discovered that the plastic coating on the carton/flap could be heated more quickly, and the overall speed of the cartoning line could thus be increased. Also, since the paperboard and its coating had been improved, the number of steps prior to applying pressure to seal and close the carton, especially with regard to preheating, could be reduced. A leader in this technology at the time is represented by the Hittenberger et al. U.S. Pat. No. 3,340,777, also owned by the present assignee. In this regard, the hot air nozzle resembled a two sided curved blade and with orifices on both sides to direct a plurality of hot air streams against the carton body and the flap, as the inverted V formed by the flap and the carton moved over the nozzle.
Later, as the technology of the coated paperboard cartons advanced further, and especially with regard to providing a carton that could be heated in the oven, further changes were made. During this era, it became common place to have the carton coated with a thermosetting resin, to allow reheating by the consumer. To obtain high speed, reliable sealing/closing, a quick drying adhesive was sprayed on the carton/flap in order to provide a rapid bonding, and thus hold the flap against the carton during the extended time that it takes for the activated thermosetting adhesive to fully bond. When this innovation came along, the same general approach to the use of a nozzle with a tapered cross section to fit within the inverted V was retained. The heat activating air streams for pre-drying the sprayed on adhesive, as well as activating the thermosetting adhesive remained basically the same. This improvement approach is shown in the Baker U.S. Pat. No. 4,249,978, and is also owned by the present assignee.
During the two decades of development represented by these prior art patents, the speed of operation, including the carton feeding and heat sealing/closing, gradually increased. However, by today's standards, the cartons were still being fed at a relatively low speed by the lugged chains running along the base of the conveyor. Given those prior art speed conditions, it was not exceedingly difficult to activate the film to form an adhesive, since the carton closer system was severely limited in speed by the turning operation on the lugged conveyors. The relatively slow speed meant that each carton/flap was simply retained in the adhesive activation zone long enough for the desired softening effect of the film or coating to provide a reliable seal.
Until just recently, the heated air nozzles illustrated in the '777 and the '978 patent had the capability of supplying sufficient heated air given the residence time of the carton within the activation zone position. In fact, before the lugless closing concept arrived on the scene, the specific approach in these two patents was generally recognized in the industry as being better than any other known methods. However, with the lugless closer technology came the need for the speed of the adhesive application portion of the operation to be increased.
It has been found in fact that it was no longer possible to obtain the type of adhesive activation of a pre-applied coating/film that is necessary for reliable sealing in a lugless closer system. The dwell or residence time was simply too short to successfully accomplish a reliable seal. Thus, in the recent past with the advent of the lugless closers, the systems were limited to the use of separately applied glue, such as by hot melt glue applicators. But, with the use of these glue applicators, a customer could still not take full advantage of the increased speed. Also, the added initial cost the cost of the glue supply and the maintenance of the system is very significant. Furthermore, some customers simply prefer cartons sealed by hot air activated adhesive from the coating/film of the carton.
Also, during this prior two decade development of the technology of hot air heat sealing, the accepted practice for stopping the heated air from being introduced along the feed path during the time when the feed of the cartons was interrupted, was to simply pivot the entire heater and nozzle assembly downwardly below the conveyor. In this manner, the nozzle was moved away from the carton flow; that is, specifically removed from the inverted V between the carton and the flap being sealed. While this prevented the carton from being heated to the point of reaching its ignition point, the heated air streams from the two sides of the nozzle continued to heat the underside of the conveyor.
By radiation, conduction, and especially by convection, as the heated air was prone to move through and up along the conveyor parts, substantial conveyor damage resulted in many cases. As a consequence, except for very short interruptions in the flow of cartons, the heating elements also had to be turned off. Even so, over time, this heating of the conveyor by residual or unwanted heat provided significant deterioration of the belts and rollers (usually plastic), a breakdown of lubricants, and even damage to sensitive adjacent electronic components. Thus, apart from not being able to find a way to supply the high volume of heated air to provide the necessary adhesive activation during the short residence time of the carton in the activation zone along a high speed cartoning line, the engineers found the problem of excessive heating of the conveyor to also be insurmountable. The continuous discharge of the high volumes of high temperature air between cartons, and especially discharge under the conveyor during these and longer periods of interruption of the carton feed, made the prior systems undesirable given the need to maintain proper conveyor integrity.
Also, due to the requirement for turning the heating elements on and off, and particularly due to the impact of the heaters as they are rapidly retracted from the feed path, greatly reduces the expected life of the delicate electric coil heating elements. In addition of course, if the heating elements have to be turned off and then back on, valuable time is lost waiting for the required temperature to be reached again and stabilized.
Accordingly, an important need is identified for taking the next step in the development of carton heat sealing/closing systems. Of particular importance is a need for improving the activation of the modern thermoplastic coating/film on cartons at a speed commensurate with the advances that have been made in other areas along the cartoning line. Of particular interest in this respect is the improvement in the speed of a related carton feeding and turning apparatus and method set forth in the co-pending U.S. Patent application, Landrum et al., Ser. No. 08/372,536, filed Jan. 13, 1995 and also owned by the assignee of the present invention. In this prior application, I, and the other inventors, have illustrated and claimed a successful lugless carton feeding/turning operation that is capable of handling cartons up to the speed of 200 cartons per minute, while at the same time maintaining the desired simplicity of the machine. The need thus applies not only to matching of the speed of this lugless turning concept, but also to provide a superior heat seal in the process, and at a significant savings in time and in the initial cost of the machine. As will be apparent, this need further calls for an advancement to deal with the substantial additional heat capacity needed to activate the adhesive in the shortened residence time in the activation zone, and at the same time to eliminate any build-up of heat along the conveyor.